The present invention relates to a process for the preparation of naphthalene carboxylic acids, in particular 2,6-naphthalene dicarboxylic acid.
2,6-naphthalene dicarboxylic acid is used as a starting material for synthesizing polyesters or polyamides useful as films or textile materials.
In the past, a number of improved processes for preparing naphthalenedicarboxylic acids (referred to hereinafter simply as NDC) have been proposed wherein diisopropylnaphthalene (referred to hereinafter simply as DiPN) is oxidized with molecular oxygen. The objects of the improvements are roughly classified as directed to one of two problems; instability of naphthalene nucleus in DiPN and the process of conversion of the isopropyl group into carboxyl group. The improvement in the former problem is directed to the instability of naphthalene nucleus itself. As the nucleus-substituted alkyl groups are not limited to an isopropyl group, the improved techniques utilize the alkyl-substituted naphthalenes. Typical improved techniques are the low temperature oxidation process as disclosed in Japanese Patent Publn. No. Sho. 48-27318, the two-step temperature elevation oxidizing process as disclosed in Japanese Patent Publn. No. Sho. 59-13495, and the oxidation process for a low concentration of the starting material as disclosed in Japanese Patent Publn. No. Sho. 56-3337. Even if these oxidation processes are utilized directly for the oxidation of DiPN, they are not effective for the preparation of NDC, so far as the latter problem residing in the conversion of isopropyl group is not solved.
A process employing a large amount of the catalyst has been proposed as represented by Japanese Laid-open Patent Appln. Nos. Sho. 60-89445, Sho. 60-89446 and Japanese Laid-open Patent Appln. No. Sho. 61-140540 which seems to be a collective edition of these references. This latter approach used the catalyst in an amount of about 10 times larger than in the conventional method and succeeded in increasing the yield of NDC on the assumption that when DiPN is oxidized under the conventional conditions for using a catalyst for oxidation of 2,6-dimethylnaphthalene, the catalyst will temporarily be deactivated and no side-reaction will take place because the isopropyl groups are abnormally rapid in the formation of an oxidation intermediate at the initial stage of the reaction as compared with the methyl groups. This process was indeed epoch-making as the oxidation process of DiPN and the yield of NDC was greatly increased.
For a process employing a large amount of the catalyst, which is said to be typical in the conventional techniques, various methods have been proposed; for example, there are proposed methods defining the amount of catalytic heavy metal per mol of DiPN or its oxidation intermediate (Japanese Laid-open Patent Appln. Nos. Sho. 60-89446 and 61-140540), Methods defining the amount of the starting material per gram catalytic heavy metal in case of a reaction mode wherein the starting material for oxidation is continuously or semicontinuously supplied to the reaction system (Japanese Laid-open Patent Appln. No. Sho. 61-140540), and methods defining the amount of DiPN and/or its oxidation intermediate per gram atom of the catalytic heavy metal in the oxidation reaction mixture (Japanese Laid-open Patent Appln. Nos. Sho. 60-89445 and 61-140540).However, these were all definitions for the amounts and did not address reaction rate.
The use of a process employing a large amount of the catalyst certainly increases the yield of NDC. However, it is a matter of course that a reaction other than a batchwise,. for example, a semi-batchwise reaction wherein the starting material is continuously and/or semi-continuously supplied a continuous process wherein the starting material is continuously and/or semi-continuously supplied and the oxidation reaction mixture is continuously and/or semi-continuously taken out, necessitates consideration of rate of supply of the starting material Although known that the semi-batchwise or continuous reaction wherein the starting material is continuously and/or semi-continuously supplied is preferable to maintain the concentration of an oxidation intermediate at a lower level, a concrete means for controlling it has not previously been proposed.
In addition, it was practically impossible to control the amount of DiPN and/or an oxidation intermediate per gram of the catalytic heavy metal element in the oxidation reaction mixture during the pressure reaction. In short, the relation between the conditions in the reactor and the velocity of supply of the starting material capable of industrial application in the semi-batchwise or continuous reaction was not as yet clear for a process employing a large amount of the catalyst.
When the reaction is carried out in the semibatchwise mode, eventually when employing a large amount of the catalyst., there is a problem that only a result similar to the batchwise reaction is obtained if the rate for supplying the starting material is excessively great. Further, there is also a problem that producibility is reduced without perceiving that unnecessary large amount of the catalyst is used in proportion to the amount of the starting material treated.
One of the problems encountered in the preparation of NDC by oxidation with molecular oxygen is the ring-opening reaction of naphthalene which forms large amounts of phthalic acid derivatives and trimellitic acid (referred to hereinafter simply as TMA),thus causing insolubilization and deactivation of the catalytic heavy metals or coprecipitation of NDC with byproducts.
Roughly two alternative methods have been proposed to solve the foregoing problem. One of them comprises weakening the acidity of the oxidation reaction solvent to inhibit side-reactions and uses as the solvent propionic acid (Japanese Laid-open Patent Appln. No. Sho. 62-120342), butyric acid, valeric acid and benzoic acid (Japanese Laid-open Patent Appln. No. Sho. 62-120343). The other method takes into consideration the composition of the catalytic heavy metals and uses cobalt (Co)/nickel (Ni)/bromine (Br) catalyst (Japanese Laid-open Patent Appln. No. Sho. 62-212343) and cobalt (Co)/cerium (Ce)/bromine (Br) catalyst (Japanese Laid-open Patent Appln. No. Sho. 62-212344). In the method reducing the acidity of the solvent, side-reaction is inhibited to a certain degree but the solvent used is more expensive than acetic acid which is widely used. In addition, the stability to oxidation of the solvent per se is low and the combustion loss is great. In the method taking into consideration on the composition of catalyst, the oxidation power of the catalyst per se is eventually weakened so that the production rate of TMA is somewhat decreased but the yield of NDC is not increased. Thus, the NDC product is contaminated with intermediates and the cost of the catalyst is increased.
As the production of TMA as by-product is thus unavoidable, there is an idea of producing both NDC and TMA originally has been proposed (Japanese Laid-open Patent Appln. No. Sho. 62-212340).
In order to enhance the yield of NDC at a lower catalyst concentration, there is known a process wherein a mixture of an aliphatic carboxylic acid and chlorobenzene, bromobenzene or nitrobenzene is used as the reaction solvent (Japanese Laid-open Patent Appln. No. 62-255448). However, the benzene derivative used in this process is a strong solvent for the catalyst, the dissociation of DiPN and the catalyst is disturbed to make the yield of NDC dissatisfactory.